To say the school massacre in Littleton, Colorado, comes as bad timing for the National Rifle Association is an understatement. Next week the powerful gun lobby will hold its annual convention in Denver, only a few miles from Columbine high school where two masked youths shot dead 12 students and a teacher on Tuesday.

Denver's mayor, Wellington Webb, has urged the NRA to cancel an event that is expected to attract 22,000 gun enthusiasts, rather than scale back the meeting, as is now planned.

'Every day, people can make a difference,' Mr Webb said after the NRA announced that out of deference it would hold the meeting on one day, May 1, rather than three. 'They can call the NRA. Get the 800 [free] number off the billboards and call them and tell them not to come to Denver.'

The NRA said exhibits, festivities, seminars and lunches surrounding the main event would be cancelled.

The tragedy 'calls upon us to modify our schedule to show our profound sympathy and respect for the families and communities in the Denver area in their time of great loss', the NRA's president, the film actor Charlton Heston, said in a letter to members.

The letter's tone was markedly different from the one Mr Heston took earlier. In a television interview he blamed the parents of the two killers, adding that the presence of an armed guard on campus might have saved the lives of those gunned down.

But the NRA has never been known for its subtlety, doling out considerable amounts of money to politicians to ensure its case receives a sympathetic hearing. In 1997-1998, the NRA channelled $1.35m (£820,000) to Republican members of Congress and $283,000 to Democrats in campaign contributions.

Although congressional members have proposed more gun control bills than ever this year, they can expect to make little headway as long as Republicans control both houses.

No major gun control initiative has been passed since the party won control of Congress in 1994. The last serious reverses suffered by the NRA were the Brady bill, which required police checks on people buying guns, and a ban on semi-automatic weapons.

The NRA has suffered setbacks outside Washington. Earlier this month voters in a Missouri referendum defeated a proposal to allow people to carry concealed weapons by 52%-48%, despite $3.8m of NRA advertising.

After the Littleton massacre, other states are putting the brakes - if only temporarily - on NRA-backed initiatives. The speaker of the Florida house of representatives withdrew a bill to protect gun makers from lawsuits arising from shootings.

In Colorado, state legislators withdrew a bill to make it easier for gun owners to get permits to carry concealed weapons. NRA-backed 'pre-emptive legislation' that would have let state law override local ordinances on gun control has also been shelved.

Before Tuesday's massacre the NRA had targeted Colorado as a state where it stood a good chance of easing regulations on guns. Now it will have to think again.